There are many different types of nozzles for spraying liquids. One type is a so-called air assisted atomizing spray nozzle. Such nozzles are capable of delivering a liquid in a finely divided, or atomized state. Atomization of a liquid in this type of nozzle is assisted by introducing air into the nozzle. More specifically, a liquid stream and an air stream are injected into a mixing chamber. The interaction of the air and liquid stream, among other factors, atomizes the liquid stream for discharge through an exit orifice of the nozzle.
Air assisted atomizing spray nozzles are used to apply agricultural chemicals and in other applications, such as pest control, where it is important to achieve a uniform distribution of relatively small amounts of chemicals. They also are used in humidifying systems to assure rapid evaporation of water into the atmosphere. Another use is in scrubbing systems for coal furnaces where rapid and complete chemical absorption of sulfurous gases must be optimized. In general, this type of nozzle is used in a wide variety of applications where it is important to deliver liquid in a finely atomized state.
One design for air assisted atomizing spray nozzles is shown in U.S. Pat. No. 5,082,185 to W. Evans. The nozzle shown therein is used with a hand-held spray gun which is particularly useful for applying pesticides. The air source for the gun can be either a high pressure tank or a tank which is pressurized by a hand pump. The design of the spray gun offers significant advantages, especially in that it reduces liquid leakage during shut-off. Nevertheless, the nozzle assembly shown in the above-mentioned Evans patent has room for improvement in several respects.
First, certain components of the nozzle are relatively fragile, in particular, the parts that define the mixing chamber 15 shown in FIG. 2 of the Evans patent. The spraying equipment, with which the nozzle is used, typically is carried from location to location. It also is carried by hand as pesticide is applied at a particular location. Under such circumstances, and even during assembly and repair of the nozzle, the perforated annular disk-shaped structure at the downstream end of the mixing chamber 15 may be bent or broken. Such damage, of course, can interrupt or diminish the performance of the spray nozzle.
Although durability may be less important if the nozzle is a component of a system which is more or less permanently installed, there are other problems with the design of the nozzle shown in the Evans patent. Nozzle parts very commonly are manufactured from cast or machined metal plugs, such as brass or stainless steel, which then are drilled or milled to provide the various openings and cavities. There are, however, a number of close-tolerance drillings which must be performed in order to form the mixing chamber 15 illustrated in the Evans patent. Consequently, manufacturing parts is relatively difficult and costly, and there are relatively high rejection rates during the manufacturing process.
In certain applications a spray nozzle will be used to spray highly abrasive liquids, such as limestone slurries in a smoke stack scrubbing system. Under such conditions the mixing chamber parts are subject to considerable wear. It is possible to increase the wear resistance of nozzle parts by using more wear resistant compositions, such as ceramics, but such materials must be cast or molded and cannot be readily machined. The mixing chamber part of the Evans patent, as a practical matter, cannot be adapted for use in high wear applications because the relatively complex design does not lend itself easily to casting or molding processes.
Moreover, it generally is desirable to minimize the quantity of air used to achieve a given degree of atomization of a given quantity of liquid. Improved air efficiency can permit the use of less expensive, lower capacity equipment and can lower operating costs in many systems. Air efficiency is especially important in equipment, such as that shown in the Evans patent, which relies on a portable air source. For example, the life span of high pressure tanks decreases as air consumption increases, and tanks have to be changed more frequently. If a hand pumped tank is used, work must be interrupted more frequently so that the tank can be pumped up.
The atomization process in this type of spray nozzle also is relatively inefficient because it relies on what may be called "parallel flow" of liquid and air. As can be seen best in the front elevational view of part 15 of the Evans patent, which view is shown in FIG. 2 and labeled 15a, the air streams and liquid streams are introduced into the mixing chamber parallel to each other. In other words, the liquid stream is introduced through the center aperture in part 15, and air is introduced through the four apertures radially disposed from the center hole but opening parallel to it.
One general approach to increasing the efficiency of the atomization process in mixing chambers has been to provide so-called impingement surfaces. Air assisted atomizing spray nozzles comprising impingement surfaces are shown, e.g., in U.S. Pat. No. 4,899,937 to J. Haruch, U.S. Pat. No. 4,815,665 to J. Haruch, and U.S. Pat. No. 4,349,156 to J. Haruch. In general, these types of designs inject a liquid stream and an air stream into a mixing chamber perpendicular to each other with an impingement surface being situated at or near the point where the streams intersect.
While this can create considerable turbulence, thereby improving the atomization process, the nozzle is more complex because it incorporates impingement surfaces. Generally, additional parts must be fabricated in order to provide an impingement surface. The relative alignment of the air inlet, liquid inlet, and impingement surface also must be relatively precisely controlled. As a consequence, it is more difficult and costly to manufacture nozzles of this type.